In recent years, because of the fast development in electronic technology, computer have become faster in operation and more compact in structure. Because they are high performance, low cost and easy to carry, portable personal computers have become more and more popular. In recent years, various types of notebook personal computers have been disclosed to greatly reduce space occupation. A notebook personal computer may be as small as 5 cms in thickness. According to the known structures, a notebook personal computer is generally made in a fixed structure, i.e., the mainframe, the keyboard and the related peripheral equipments are fixedly connected into a unitary device. In some occasions, one may need to use only part of the whole assembly of a portable personal computer. However, because a portable personal computer is not detachable, the whole assembly must be taken along even if only a part of the computer is to be used.